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Editorial | Previous Editorials
June 2004


Clinical Significance of Mucin Balls

Eric Papas PhD, MCOptom, DCLP

Eric Papas is Executive Director of Research & Development at the Vision Co-operative Research Centre, based at the University of New South Wales, Sydney, Australia. He originates from England where he obtained degrees in physics and optometry, before spending several years in specialist contact lens practice. Most of his subsequent career has been devoted to research and he has managed clinical research groups for corporations such as Hydron and Allergan. More recently he was Director of Clinical Research in the Cornea and Contact Lens Research Unit (CCLRU) at UNSW. His latest role involves supervision of a range of vision correction projects that are either government funded or run in direct collaboration with industrial partners. Currently, his major research interests are the tear film, ocular surface sensation and vascular behaviour.


Mucin Balls….I always think they are a bit like the Hilton sisters, Paris and Nikki……. everyone has seen them, no-one is particularly impressed, but you vaguely wish they’d go away! These round, translucent or opalescent bodies (Figure 1) can be seen trapped in the space between the contact lens and cornea and are usually around 50µm in diameter, though size can vary between about 20 and 200µm. While much more common among wearers of silicone hydrogel lenses, individual susceptibility varies substantially.

Figure 1: Contact lens wearer displaying large numbers of mucin balls

In assessing the significance of these features, the first task for the clinician is to make sure that they are indeed looking at mucin balls, rather than the features with which they are most commonly confused, namely microcysts or vacuoles. Among the signs to look for when making the differential diagnosis are that mucin balls have a pre-epithelial location, and will usually disappear within a few blinks after lens removal. Both microcysts and vacuoles are intra-epithelial, and persist when the lens is removed. If doubt still remains, instillation of sodium fluorescein will show pooling of the dye (though not frank staining), in the epithelial depressions left behind by the mucin ball (Table 1). This appearance is similar to “dimple veiling”, which was caused by bubbles of gas collecting beneath rigid lenses.

Mucin Balls



Abolished on lens removal

Persist on lens removal


Reversed Illumination

Unreversed Illumination

NaFL pooling in depression

Occasional NaFl stain

No NaFl stain

Table 1: Differential diagnosis of mucin balls

Our knowledge of the nature of mucin balls has improved recently as samples have been collected and subjected to scanning electron microscopy, histochemistry and other analytical techniques.3 These studies have shown it to be quite unlikely that mucin balls have been misnamed, which is to say that their major constituent does indeed appear to be mucin! Presumably, this material is derived from the pre-ocular tear film and is formed into spheroids by the relative motion between the contact lens and corneal surface. Consequently, it has been proposed that reducing this motion, by more closely matching lens shape to that of the ocular surface, would be a possible means of cutting down mucin ball numbers.1

Despite being squeezed between the contact lens and epithelium, mucin balls tend to maintain their spherical shape. This resistance to deformation is responsible for creating the epithelial depressions and these may be quite deep. When viewed with the confocal microscope, the indentations can often be seen extending at least to the level of Bowman’s membrane4, and possibly even beyond.5

In these circumstances one might reasonably wonder what happens to the corneal cells immediately beneath a mucin ball? So far, it has proved difficult to visualize epithelial cells in this region; suggesting either, that they have become highly compacted, or possibly that migration away from the local pressure point has occurred. Interestingly, a similar conundrum currently surrounds the mechanisms of epithelial remodeling during orthokeratology, and in that case, recent data tend to favour the compaction theory.6

With respect to stromal tissue, indications from rabbit eyes are that the keratocytes or fibroblasts underneath deeper mucin ball impressions show signs of increasing their proliferation rate.4 The significance of this finding currently remains unclear, though stromal cells usually divide rather slowly.

While evidence of cellular activity in the cornea is certainly a good reason for continued research, from the viewpoint of both clinician and wearer, it is comforting that there have been almost no reports of associated negative clinical events. This is despite the fact that mucin balls are highly prevalent among silicone hydrogel wearers. Apart from a statistical indication that extended wearers with larger numbers may have a slightly increased risk (x 1.7) of developing a contact lens induced peripheral ulcer (CLPU),7 mucin ball related adverse events have not featured in the literature. As there are generally no adverse effects on either vision or subjective comfort,1, 8, clinical intervention is usually unnecessary in cases where mucin balls are the only observable complication. Naturally however, continued, regular review is recommended throughout the period of contact lens wear.

  1. Dumbleton K, Jones L, Chalmers R, Williams-Lyn D, Fonn D. Clinical characterization of spherical post-lens debris associated with lotrafilcon high-Dk silicone lenses. CLAO J. 2000; 26:186-92.
  2. Sweeney DF, du Toit R, Keay L, Jalbert I, Sankaridurk PR, Stern J, Skotnitsky C, Stephensen A, Covey M, Holden BA & Rao G. Clinical performance of silicone hydrogel lenses. In: Silicone Hydrogels, continuous wear contact lenses. Ed: Sweeney DF. 2nd Edition, Butterworth Heinemann. p 198.
  3. Miller TJ, Papas EB, Ozkan J, Jalbert I & Ball M. Clinical Appearance and Microscopic
    Analysis of Mucin Balls Associated with Contact Lens Wear. Cornea, 2003, 22,, 740-745.
  4. Ladage PM, Petroll WM, Jester JV, Fisher S, Bergmanson JPG, Cavanagh HD. Spherical indentations of human and rabbit corneal epithelium following extended contact lens wear. CLAO J. 2002; 28:177-180.
  5. Jalbert I, Stapleton F, Papas E, Sweeney DF & Coroneo M. In vivo confocal microscopy of the human cornea. Brit. J. Ophthalmol. 2003; 87: 225-236.
  6. Choo J, Caroline PJ, Harlin DD & Myers W. Morphological changes in the cat epithelium following overnight lens wear with the Paragon CRT for corneal reshaping. Invest Ophthalmol. Vis. Sci. 2004, 45, Abstract 1552.
  7. Naduvilath TJ. Statistical modeling of risk factors associated with soft contact lens related corneal infiltrative events. PhD Thesis (Newcastle: University of Newcastle), 2003.
  8. Morgan PB & Efron N. Comparitive clinical performance of two silicone hydrogel contact lenses for continuous wear. Clin Exp Optom., 2002; 85, 183-192.

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